This invention relates to derivatives of the cyclodextrins (which are cyclic oligosaccharides), inclusion complexes comprising cyclodextrin derivatives, and to processes for preparing and using the derivatives and complexes. This invention also relates to cyclodextrin derivatives useful as synthetic intermediates, and to processes for preparing such intermediates. This invention also relates broadly to pharmaceutical, pesticidal, herbicidal, agricultural, cosmetic or personal care agent compositions comprising the cyclodextrin derivatives and inclusion complexes. This invention further relates to pharmaceutical compositions, and to processes for treating animals, including humans. This invention also relates to chromatographic compositions, and to a method for separating racemic mixtures.
Inclusion complexes are chemical species consisting of two or more associated molecules in which one of the molecules, the "host", forms or possesses a cavity into which it can admit a "guest" molecule, resulting in a stable association without formation of any covalent bonds. Secondary forces are alone responsible for maintenance of the integrity of all inclusion complexes.
Over the past twenty-five years, interest in the physical and chemical properties of inclusion complexes has grown considerably. The cyclodextrins, because they are nontoxic and are able to form complexes with numerous small organic molecules, perhaps the most important of all compounds capable of acting as host components. The three most important cyclodextrins are the .alpha.-, .beta.- and .gamma.-cyclodextrins, which respectively consist of six, seven and eight .alpha.-D-gluco-pyranosyl residues. Cyclodextrins are not perfectly cylindrical molecules, but are somewhat coneshaped, with all of the secondary hydroxyl groups situated at one end of the annulus (the larger diameter end) and all of the primary hydroxyl groups at the other, as illustrated in FIG. 1.
Each glucopyranose residue contains three hydroxyls which are of varying reactivity. The primary or C6 hydroxyl is the most reactive by virtue of its position on the C6 carbon, which is the only primary carbon. The secondary or C2 and C3 hydroxyls are less reactive because of their location on the C2 and C3 carbons, which are secondary carbons. Thus, synthetic reactions will usually involve one or more of the C6 hydroxyls unless steps are taken to proceed via the C2 and/or C3 hydroxyls.
The numbering system and structure of .beta.-cyclodextrin are shown in FIG. 2. Viewing the secondary end of the cyclodextrin, each glucopyranosyl residue is labelled clockwise from A to F, G or H (for .alpha.-, .beta.- and .gamma.-cyclodextrins, respectively). The A residue is determined by the substitution which takes priority. Thus each substituent is assigned a prefix indicating the number of the carbon to which the substituent is attached and the letter of the glucopranose residue. For example, 6.sup.A -o-toluenesulfonyl-.beta.-cyclodextrin indicates that a toluenesulfonyl is attached to the oxygen on the C6 carbon of the A-glucopyranose residue.
The cavity of a cyclodextrin is relatively apolar and thus cyclodextrins will normally preferentially include apolar or hydrophobic molecules (or portions thereof) in their cavities, while excluding polar or hydrophilic molecules or portions thereof. The molecular dimensions and solubility of .alpha.-, .beta.-, and .gamma.-cyclodextrin are provided in Table 1.
TABLE 1 ______________________________________ Physical Properties of Cyclodextrins Number of Solubility Cavity D-glucosyl Molecular in water width residues Name weight (g/100 mL) (pm) ______________________________________ 6 alpha 972 14.5 470-520 cyclodextrin 7 beta 1135 1.85 600-640 cyclodextrin 8 gamma 1297 23.2 750-830 cyclodextrin ______________________________________
As shown in Table 1, the space within the cyclodextrin cavity increases with the number of D-glucopyranosyl residues. Thus, as would be expected, the stability of an inclusion complex depends to a large degree on the relative sizes of the cyclodextrin cavity and the portion of the guest molecule to be included. For example, a molecule may be too large to fit within the cavity of .alpha.-cyclodextrin, but might form a complex with the larger .beta.-cyclodextrin. A further discussion of the molecular interaction of inclusion complexes is provided in R. J. Clarke, J. H. Coates and S. F. Lincoln, "Inclusion Complexes of the Cyclomalto-Oligosaccharides (Cyclodextrins)", Advances in Carbohydrate Chemistry and Biochemistry, Vol 46, pp 205-249 (1988), which is expressly incorporated herein by reference.
Cyclodextrins have been widely investigated for use in the pharmaceutical industry because of their reported ability to solubilize and stabilize drugs and to increase bioavailability. This ability has led to many scientific publications, and numerous worldwide patents and published applications relating to the use of cyclodextrins with pharmaceuticals. See, for example, U.S. Pat. No. 4,727,064 to Pitha and U.S. Pat. No. 4,707,472 to Inagaki et al., U.S. Pat. No. 4,774,329 to Friedman, and U.S. Pat. No. 4,432,802 to Harata et al., U.S. Pat. Nos. 4,518,588 and 4,274,985 to Szejtli et al., U.S. Pat. No. 4,722,815 to Shibanai, U.S. Pat. No. 4,598,070 to Ohwaki et al., and U.S. Pat. No. 4,424,209 to Tuttle, and the references cited therein.
Despite all of this research activity, the commercial use of cyclodextrins to deliver drugs has remained relatively low for a few reasons. First, while each type of cyclodextrin can form inclusion complexes by including other molecules in the annulus, for a variety of drugs in common usage the .beta.-cyclodextrin annulus is the most appropriate size. Although .beta.-cyclodextrin is the most readily available and least expensive cyclodextrin, it is also the lowest in solubility (1.85 g/100 ml) which substantially decreases its utility as a drug delivery system. This is especially true when the drug is itself substantially insoluble; the resulting inclusion complex generally having a solubility somewhere between the solubility of the drug and cyclodextrin. Thus, one of the main benefits of cyclodextrins, i.e., the ability to solubilize otherwise insoluble drugs, is lost when .beta.-cyclodextrin is used to include the drug.
Another main benefit of cyclodextrins, i.e., the ability to stabilize and increase the bioavailability of drugs, stems from the ability of cyclodextrins to strongly include a drug molecule. These benefits are particularly important where (1) the drug is rendered inactive by hydrolysis in the acidic environment of the stomach, which has a pH of generally less than about 3, (2) the drug irritates the gastrointestinal lining such as, for example, non-steroidal anti-inflammatory compounds, or (3) the drug has a short half-life such that it is immediately absorbed through the lining of the small intestine, and within a few hours, is no longer present in effective serum concentrations. It is the ability of cyclodextrins to form highly stable inclusion complexes which in fact prevents the drug from being freely available to undergo hydrolysis, irritate the stomach lining or absorb too quickly into the blood stream. This ability, however, depends upon whether the molecular dimensions of the drug molecule happen to fit one of the three cyclodextrins, so that a strong intermolecular attraction exists resulting in a high stability constant.
The stability constant (or association constant), K, of cyclodextrin inclusion complexes is a measure of the proportions of free drug and free cyclodextrins in solution versus the concentration of drug-cyclodextrin inclusion complex (or included drug). ##EQU1##
When the stability constant is high only a small proportion of the drug is in the free state in the gastrointestinal tract. Thus only a small proportion of the drug will be exposed to hydrolysis and will be available to cause gastrointestinal irritation, and in the case of a drug of low solubility, to precipitate. However, as the drug is absorbed in the small intestine, further drug will be rapidly released in a controlled manner, from the drug-cyclodextrin inclusion complex, so that a continuous absorption of the drug by the small intestine occurs.
Obviously, when the stability constant is low the proportion of the drug free in the gastrointestinal tract will be high. This exposes a large proportion of the drug to hydrolysis. A large proportion of the drug is also available to cause irritation of the gastrointestinal tract. If the free drug is of low solubility, precipitation of solid drug will occur in the gastrointestinal tract.
While there have been some fairly high stability constants reported (e.g., up to 3.7.times.10.sup.4 for some .gamma.-cyclodextrin-steroid systems; Uekama et al., Int. J. Pharmaceuticals, Vol. 10 (1982) pp 1-15), most known cyclodextrin-drug inclusion complexes have stability constants only on the order of from less than about 10.sup.3 M.sup.-1 up to about 10.sup.4 M.sup.-1 (especially for smaller drug molecules), and it is for this reason that cyclodextrins have not gained widespread popularity. Moreover, given the wide range of molecular sizes exhibited by drug molecules, only a fraction of the drugs needing a superior delivery system will be of correct size to form an inclusion complex of high stability with one of the three naturally occurring cyclodextrin molecules.
It is apparent that high stability constants are required to ensure a high proportion of included drug molecules and thus why cyclodextrins have yet to achieve widespread commercial use. A perspective of stability constants can be gained from the following model, which follows Habon, et al., J. Pharmazie, Vol 39, pp 830-834 (1984) and assumes a drug solubility of 10.sup.-4 M (relatively insoluble) and a 1:1 drug/CD complex solubility of 10.sup.-3 M, with a total of 10.sup.-4 mole of drug/CD complex present. If one assumes a stability constant of 10.sup.4 M.sup.-1 (relatively high) and a total volume of solution of 100 ml, (approximately an unfilled human stomach, e.g., before a meal), 40% of the drug would be present as undissolved (i.e., insoluble) solid drug, 10% as dissolved drug and only 50% as dissolved complex. For a stability constant of 10.sup.4 M.sup.-1 and a total volume of 1000 ml (approximately a filled human stomach), 62% of the drug would be present as dissolved pure drug and only 38% as a dissolved drug/CD complex. For a stability constant of 10.sup.2 M.sup.-1 and a total volume of 1000 ml, 99% of the drug would be present as dissolved pure drug and only 1% a drug/CD complex.
In attempts to overcome these problems, researchers have tried using either modified cyclodextrins, which have groups or pendant arms substituted onto the cyclodextrins, or polymeric cyclodextrins which generally comprise either randomly polymerized cyclodextrins, or cyclodextrins randomly attached to a preformed polymer backbone.
There have also been other attempts to prepare modified cyclodextrins, although the study of many such modified cyclodextrins has been confined mostly to observing interactions with dyes or other agents which are not pharmaceutical, pesticidal, herbicidal or agricultural. Moreover, the descriptions of the synthetic procedures in some instances appear insufficient to enable others of ordinary skill to reproduce the reported work.
In the case of modified cyclodextrins, those that are currently available are characterized by having some or all of their hydroxyl hydrogens replaced by methyl, ethyl, hydroxyethyl or hydroxypropyl. Such modifications lead in some cases to enhanced solubility of the cyclodextrin and its inclusion complexes formed with poorly soluble drug molecules, but do not address the problem of increasing the drug-cyclodextrin stability constants. Moreover, many of these modified cyclodextrins are the products of modifications which can be controlled only to a limited extent, and which yield a relatively inseparable mixture of mono-, di- and other randomly multisubstituted cyclodextrins. The cyclodextrin product is thus identifiable only in terms of a "degree of substitution." In the case of cyclodextrin polymers, it can also be likewise difficult to control the reaction and thus one cannot generally be certain of the product. Moreover there is also a problem with efficiency, i.e., large amounts of cyclodextrin are required to deliver only small amounts of included drug, resulting in the patient having to swallow many capsules or tablets to obtain a normal dose of a drug.
Another type of modification has been the covalent linking of two cyclodextrin molecules as reported by Fujita et al., J. Chem. Soc., Chem. Commun., (1984) 1277-1278, and Chemistry Lett., (1985) 11-12; Harada et al., Polymer J., Vol. 12 (1980) pp 29-33; and Tabushi et al., J. Am. Chem. Soc., Vol. 101 (1979) pp 1614 et seq. However, no further works has been performed to determine whether linked cyclodextrins are useful for including pharmaceuticals or other useful agents which can have intended bio-affecting activity, for example, pesticides, herbicides and agricultural products.
In a modification reported by Tabushi et al., J. Am. Chem. Soc., Vol. 108 (1986) 4514-4518, .beta.-cyclodextrin was doubly substituted with amine and carboxylic acid pendant arms. These functionalities allowed for an increased association with the amino acid, tryptophan, which was included in the cyclodextrin annulus. Again, there have been no attempts to form such associations with, for example, associable groups of pharmaceutical agents for delivering drugs into human or other host animals.
Kojima et al. reported covalently bonding the vitamin nicotinamide to a cyclodextrin. Tetrahedron Lett., Vol. 21 (1980) 2721. However, the resulting cyclodextrin derivative would not have been suitable for delivery of the vitamin to a host animal because the tertiary amine bond created between the nicotinamide and the cyclodextrin, if broken, would not yield the nicotinamide in active form. Moreover, that bond would not break in a host animal under the conditions normally found, for example, the acidic conditions of the stomach.
In summary, despite the large volume of research relating to modified and unmodified cyclodextrins, there remains a long felt need for cyclodextrin derivatives and inclusion complexes which have good solubility properties and which can deliver problem drugs in a pharmacologically acceptable manner. This long felt need has been accompanied by the need for processes and intermediates by which such derivatives and complexes can be readily prepared.
It is against this background that the present invention is brought forth. While much of the foregoing discussion has dealt with the use of cyclodextrins to deliver drugs, those skilled in the art, upon reading this disclosure, will recognize many other commercial applications for this invention such as, for example, delivery systems for pesticides, herbicides, agricultural products such as feeds and fertilizers, and food additives, to name only a few.